The Lock number is a dimensionless parameter for aircraft rotor blades, and the equation isγ=ρacR4/Ib,where                γ=Lock number        ρ=air density        a=slope of the 2-D airfoil lift curve        c=chord length        R=rotor radius        Ib=flapping moment of inertia.The Lock number represents the ratio of aerodynamic forces, which act to lift the blade, to inertial forces, which act to maintain the blade in the plane of rotation. Typical rotorcraft blades have a Lock number of between 3 and 12.        
The inertial forces are based on the mass of each blade, so a larger blade tends to have a lower Lock number. For example, a two-blade helicopter rotor typically has blades with high inertia, and this is due to the size of each blade required to achieve the desired amount of lift. However, a rotor can provide the same or more lift by using a larger number of smaller and lighter (higher Lock number) blades. This reduces the mass and total inertia of the rotor and reduces the loads that must be reacted by the rotor hub, allowing for a lighter hub. Another advantage to reducing rotor mass and inertia is that the jump-takeoff load, which is used to design the roof structure, increases with rotor inertia. In addition, reducing the mass of the rotor reduces the load it applies to the fuselage in a crash. Therefore, reducing the mass of the rotor may allow for a lighter fuselage design, with fuselage mass perhaps being reduced by twice the amount removed from the rotor.
Another engineering consideration is that the combined inertia of the blades be high enough to allow for autorotation after engine failure, so single-engine aircraft typically have high-inertia rotors, whereas multi-engine aircraft can use rotors with less inertia. One way to achieve higher inertia is to add tip weights to the blades, but another way is to add blades to the rotor. As described above, adding narrower, lighter blades with a higher Lock number can allow for an aircraft with reduced weight in both the rotor system and the fuselage.
Using an increased number of narrower blades has other advantages. One advantage is that reducing the chord width reduces material cost for each blade, which can significantly reduce the price of a shipset of blades. Also, the rotor is quieter during operation due to the reduced blade noise, which tends to vary with chord width, and to the increased number of blade passages, which coalesce into a higher frequency and less offensive sound.